Gerrit only: Renovate doesn't properly allow specifying commit trailers through the commitBody config

[travisshivers]

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Gerrit, 39.82.3

Please tell us more about your question or problem

The commitBody configuration seems like it is designed to be able to specify commit trailers in it since renovate even has a gitSignOff preset which should add the "Signed-off-by: {{{gitAuthor}}}" trailer.

Git trailers follow specific requirements and need to be the last block of a commit message for them to actually be trailers. https://git-scm.com/docs/git-interpret-trailers

Renovate appears to properly handle this for all cases except for gerrit since it needs to insert some other trailers

renovate/lib/modules/platform/gerrit/scm.ts

Lines 118 to 121 in 8d31e5c

	commit.message = [
	...message,
	`Renovate-Branch: ${commit.branchName}\nChange-Id: ${existingChange?.change_id ?? generateChangeId()}`,
	];

Renovate adds these internal trailers as the last entry to commit.message array which simple-git then converts to a single string by joining them with two line breaks between them.

This ultimately means that renovate can't be configured to add on additional git trailers when using gerrit.

---

Example:

Renovate config

{
  "commitBody": "My-Trailer: test"
}

Ultimate gerrit commit message:

chore(deps): update mydep digest to v1.0.1

My-Trailer: test

Renovate-Branch: my-branch-name
Change-Id: I24e1e7c57c808ba81de51b118e3146336f771bba

In the commit message above, we can see that My-Trailer will not be interpreted as a trailer because it is not in the last group of lines.

---

One potential fix might be to append these renovate generated trailers to the last element of message if message has more than one element.

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